Method and system for a narrowband, non-linear optoelectronic receiver

ABSTRACT

Methods and systems for a narrowband, non-linear optoelectronic receiver are disclosed and may include amplifying a received signal, limiting a bandwidth of the received signal, and restoring the signal utilizing a level restorer, which may include a non-return to zero (NRZ) level restorer. The NRZ level restorer may include a pulse-triggered bistable circuit, which may include two parallel inverters, with one being a feedback path for the other. The inverters may be single-ended or differential. A photogenerated signal may be amplified in the receiver utilizing a transimpedance amplifier and programmable gain amplifiers (PGAs). A received electrical signal may be amplified via PGAs. The bandwidth of the received signal may be limited utilizing one or more of: a low pass filter, a bandpass filter, a high pass filter, a differentiator, or a series capacitance on the chip. The signal may be received from a photodiode integrated on the chip.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application makes reference to and claims priority to U.S.Provisional Application Ser. No. 60/998,314 filed on Oct. 10, 2007,which is hereby incorporated herein by reference in its entirety.

The above stated application is hereby incorporated herein by referencein its entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[Not Applicable]

MICROFICHE/COPYRIGHT REFERENCE

[Not Applicable]

FIELD OF THE INVENTION

Certain embodiments of the invention relate to signal processing. Morespecifically, certain embodiments of the invention relate to a methodand system for a narrowband, non-linear optoelectronic receiver.

BACKGROUND OF THE INVENTION

As data networks scale to meet ever-increasing bandwidth requirements,the shortcomings of copper data channels are becoming apparent. Signalattenuation and crosstalk due to radiated electromagnetic energy are themain impediments encountered by designers of such systems. They can bemitigated to some extent with equalization, coding, and shielding, butthese techniques require considerable power, complexity, and cable bulkpenalties while offering only modest improvements in reach and verylimited scalability. Free of such channel limitations, opticalcommunication has been recognized as the successor to copper links.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with the present invention as set forth inthe remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

A system and/or method for a narrowband, non-linear optoelectronicreceiver, substantially as shown in and/or described in connection withat least one of the figures, as set forth more completely in the claims.

Various advantages, aspects and novel features of the present invention,as well as details of an illustrated embodiment thereof, will be morefully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention.

FIG. 1B is diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.

FIG. 2 is a block diagram illustrating an exemplary narrowband receiverarchitecture, in accordance with an embodiment of the invention.

FIG. 3 is a block diagram of an exemplary narrow-band, non-linearoptoelectronic receiver system, in accordance with an embodiment of theinvention.

FIG. 4 is a plot of an input signal and the resulting output signal, inaccordance with an embodiment of the invention.

FIG. 5 is a diagram illustrating performance improvements versus filterand noise bandwidth, in accordance with an embodiment of the invention.

FIG. 6 is a diagram illustrating sensitivity penalty at a bit error rate(BER) of 1e-12, in accordance with an embodiment of the invention.

FIG. 7 is a diagram illustrating input and output signals in anarrow-band, non-linear optoelectronic receiver, in accordance with anembodiment of the invention.

FIG. 8 is a flow diagram illustrating exemplary steps for a narrow-band,non-linear optoelectronic receiver, in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of the invention may be found in a method and system fora narrowband, non-linear optoelectronic receiver. Exemplary aspects ofthe invention may comprise amplifying a received signal, limiting abandwidth of the received signal, and restoring the amplified,bandwidth-limited received signal utilizing a level restorer. The levelrestorer may comprise a non-return to zero (NRZ) level restorer. The NRZlevel restorer may comprise a pulse-triggered bistable circuit. Thepulse-triggered bistable circuit may comprise two parallel inverters,where a first of the inverters comprises a feedback path for a second ofthe inverters, and where the inverters may comprise single-ended ordifferential inverters. A photogenerated signal may be amplified in thereceiver utilizing a transimpedance amplifier and one or more variablegain amplifiers (VGAs). A received electrical signal may be amplifiedvia one or more VGAs. The bandwidth of the received signal may belimited utilizing one or more of a low pass filter, a bandpass filter, ahigh pass filter, a differentiator, and/or a series capacitance on thechip. The signal may be received from a photodiode integrated on thechip.

FIG. 1A is a block diagram of a photonically enabled CMOS chip, inaccordance with an embodiment of the invention. Referring to FIG. 1A,there is shown optoelectronic devices on a CMOS chip 130 comprising highspeed optical modulators 105A-105D, high-speed photodiodes 111A-111D,monitor photodiodes 113A-113H, and optical devices comprising taps103A-103K, optical terminations 115A-115D, and grating couplers117A-117H. There is also shown electrical devices and circuitscomprising transimpedance and limiting amplifiers (TIA/LAs) 107A-107E,analog and digital control circuits 109, and control sections 112A-112D.Optical signals are communicated between optical and optoelectronicdevices via optical waveguides fabricated in the CMOS chip 130.

The high speed optical modulators 105A-105D comprise Mach-Zehnder orring modulators, for example, and enable the modulation of the CW laserinput signal. The high speed optical modulators 105A-105D are controlledby the control sections 112A-112D, and the outputs of the modulators areoptically coupled via waveguides to the grating couplers 117E-117H. Thetaps 103D-103K comprise four-port optical couplers, for example, and areutilized to sample the optical signals generated by the high speedoptical modulators 105A-105D, with the sampled signals being measured bythe monitor photodiodes 113A-113H. The unused branches of the taps103D-103K are terminated by optical terminations 115A-115D to avoid backreflections of unwanted signals.

The grating couplers 117A-117H comprise optical gratings that enablecoupling of light into and out of the CMOS chip 130. The gratingcouplers 117A-117D are utilized to couple light received from opticalfibers into the CMOS chip 130, and the grating couplers 117E-117H areutilized to couple light from the CMOS chip 130 into optical fibers. Theoptical fibers may be epoxied, for example, to the CMOS chip, and may bealigned at an angle from normal to the surface of the CMOS chip 130 tooptimize coupling efficiency.

The high-speed photodiodes 111A-111D convert optical signals receivedfrom the grating couplers 117A-117D into electrical signals that arecommunicated to the TIA/LAs 107A-107D for processing. The analog anddigital control circuits 109 may control gain levels or other parametersin the operation of the TIA/LAs 107A-107D. The TIA/LAs 107A-107D thencommunicate electrical signals to other circuitry on the CMOS chip 130and/or circuitry/devices off-chip.

The TIA/Las 107A-107D may comprise narrowband, non-linear optoelectronicreceiver circuitry. Accordingly, the narrowband receiver front-end maybe followed by a non-return to zero (NRZ) level restorer circuit. Thiscircuit limits the bandwidth of the optical receiver in order todecrease the integrated noise, thereby increasing the signal to noiseratio. An NRZ level restorer may be used to convert the resulting datapulses back into NRZ data.

Specifically, the narrowband receiver front-end provides amplificationin a narrow band which includes the maximum data tone (i.e. 5 GHz for 10Gb/s NRZ data). This limited bandwidth may be implemented in a varietyof ways including, but not limited to, the integration of a seriesfilter, such as high pass, band pass or differentiating filters, forexample. A non-linear NRZ level restorer may then be used to convert theresulting pulses back to their NRZ levels, creating an effectively 0 Hzlow frequency cut-off. The NRZ level restorer may be implemented using avariety of techniques, including a bistable cross-coupled inverter pair,which may be pulse triggered to switch between its two states.

The control sections 112A-112D comprise electronic circuitry that enablemodulation of the CW laser signal received from the taps 103A-103C. Thehigh speed optical modulators 105A-105D require high-speed electricalsignals to modulate the refractive index in respective branches of aMach-Zehnder interferometer (MZI), for example. The voltage swingrequired for driving the MZI is a significant power drain in the CMOSchip 130. Thus, if the electrical signal for driving the modulator maybe split into domains with each domain traversing a lower voltage swing,power efficiency is increased.

FIG. 1B is a diagram illustrating an exemplary CMOS chip, in accordancewith an embodiment of the invention. Referring to FIG. 1B, there isshown the CMOS chip 130 comprising electronic devices/circuits 131,optical and optoelectronic devices 133, a light source interface 135,CMOS chip surface 137, an optical fiber interface 139, and CMOS guardring 141.

The light source interface 135 and the optical fiber interface 139comprise grating couplers that enable coupling of light signals via theCMOS chip surface 137, as opposed to the edges of the chip as withconventional edge-emitting devices. Coupling light signals via the CMOSchip surface 137 enables the use of the CMOS guard ring 141 whichprotects the chip mechanically and prevents the entry of contaminantsvia the chip edge.

The electronic devices/circuits 131 comprise circuitry such as theTIA/LAs 107A-107D and the analog and digital control circuits 109described with respect to FIG. 1A, for example. The optical andoptoelectronic devices 133 comprise devices such as the taps 103A-103K,optical terminations 115A-115D, grating couplers 117A-117H, high speedoptical modulators 105A-105D, high-speed photodiodes 111A-111D, andmonitor photodiodes 113A-113H.

FIG. 1C is a diagram illustrating an exemplary CMOS chip coupled to anoptical fiber cable, in accordance with an embodiment of the invention.Referring to FIG. 1C, there is shown the CMOS chip 130 comprising theelectronic devices/circuits 131, the optical and optoelectronic devices133, the light source interface 135, the CMOS chip surface 137, and theCMOS guard ring 141. There is also shown a fiber to chip coupler 143, anoptical fiber cable 145, and a light source module 147.

The CMOS chip 130 comprising the electronic devices/circuits 131, theoptical and optoelectronic devices 133, the light source interface 135,the CMOS chip surface 137, and the CMOS guard ring 141 may be asdescribed with respect to FIG. 1B.

In an embodiment of the invention, the optical fiber cable may beaffixed, via epoxy for example, to the CMOS chip surface 137. The fiberchip coupler 143 enables the physical coupling of the optical fibercable 145 to the CMOS chip 130.

The light source module 147 may be affixed, via epoxy or solder, forexample, to the CMOS chip surface 137. In this manner a high power lightsource may be integrated with optoelectronic and electronicfunctionalities of one or more high-speed optoelectronic transceivers ona single CMOS chip.

FIG. 2 is a block diagram illustrating an exemplary narrowband receiverarchitecture, in accordance with an embodiment of the invention.Referring to FIG. 2, there is shown a receiver architecture 200comprising a filter 201 and a non-return to zero (NRZ) bistable levelrestorer 203. There is also shown an input signal 205, a filtered signal207, and an output signal 209.

The filter 201 comprises circuitry for filtering signals of frequenciesoutside of a desired frequency band. For example, the filter may allowsignals above a frequency, f_(f), and attenuate signals below f_(f), asshown in the frequency response plot shown below the filtered signal207.

In operation, the filter 201 may limit the bandwidth of the opticalreceiver in order to decrease the integrated noise, thereby increasingthe signal to noise ratio. The NRZ bistable level restorer 203 mayconvert the resulting data pulses back into NRZ data. The NRZ bistablelevel restorer 203 may be implemented using a variety of embodimentsincluding a bistable cross-coupled inverter pair, for example, which maybe pulse triggered to switch between its two states.

In another embodiment of the invention, the NRZ level restorer 203 maycomprise a pulse-triggered bistable circuit, a Schmitt-trigger circuit,an RS flip-flop with an inverter on the R input, or a cross-coupledtransistor pair, for example. In an embodiment of the invention, thebistable circuit may comprise an inverter with a second inverter asfeedback from the output to the input, where the inverters may besingle-ended or differential.

The narrowband receiver architecture 200 may provide amplification in anarrow band which includes the maximum data tone (i.e. 5 GHz for a 10Gb/s NRZ data). This limited bandwidth may be implemented in a varietyof ways including, but not limited to, the integration of a seriesfilter, such as a high pass, band pass, and/or differentiating filter.The NRZ bistable level restorer 203 may then convert the resultingpulses back to their NRZ levels, creating an effectively 0 Hz lowfrequency cut-off.

FIG. 3 is a block diagram of an exemplary narrow-band, non-linearoptoelectronic receiver system, in accordance with an embodiment of theinvention. Referring to FIG. 3, there is shown a receiver 300 comprisinga photodetector 301, a transimpedance amplifier (TIA) 303, a variablegain amplifier (VGA) 305, an automatic gain control (AGC) block 307, afilter 309, and an NRZ level restorer 311.

The TIA 303 comprises an amplifier that is enabled to receive an inputcurrent signal and generate a voltage output signal. The photodetector301 comprises a germanium diode, for example, and may be integrated onthe same chip as the TIA 303, the VGA 305, the AGC 307, the filter 309,and the NRZ level restorer 311.

The VGA 305 comprises an amplifier that may be enabled to receive aninput signal and generate an amplified output signal, where the gainlevel may be configured depending on the desired signal level. The VGA305 may also receive as an input, an output signal from the AGC 307,which may enable the control of the gain level of the VGA 305. The AGCblock 307 may determine the signal level at the output of the VGA 305,compare to the desired level, and generate an output signal to adjustthe gain of the VGA 305 accordingly.

The filter 309 comprises a high pass filter to remove unwanted signalsbelow a cutoff frequency, such as the cutoff frequency f_(f), shown inFIG. 2. In this manner, the desired signal may be in the frequency rangeallowed to pass, while the low frequency noise signal may be filteredout. In another embodiment of the invention, the filter 309 may comprisea high-pass filter, a band-pass filter, a differentiator, or a seriescapacitance, for example. Additionally, the filter 309 may be placed atany point in the circuit from before the TIA 303 to after the lastamplifier stage, as long as it precedes the NRZ level restorer 311.

The NRZ level restorer 311 may be substantially similar to the NRZbistable level restorer 203, described with respect to FIG. 2, and maybe enabled to receive the filtered signal from the filter 309, andgenerate an output signal V_(OUT), with a restored waveform, as shown bythe output signal 209 in FIG. 2.

In operation, an optical signal may be received by the photodetector 301and converted into an electrical current, which may be converted to avoltage signal by the TIA 303. The resulting signal may then beamplified by the VGA 305 at a gain level configured by the AGC 307. TheAGC 307 may determine the signal level at the output of the VGA 305 andadjust the gain of the VGA 305 as needed. The filter 309 may filter outlow frequencies in the amplified signal with the resulting signal beingcommunicated to the NRZ level restorer 311. The NRZ level restorer mayrestore the received signal to a square wave signal, resulting in theoutput signal V_(OUT).

The limited receiver linear bandwidth decreases the integrated receivernoise, improving the signal to noise ratio and hence the sensitivity ofthe receiver, relative to a traditional receiver without the filteringand NRZ level restorer. The limited bandwidth also decreasessusceptibility to crosstalk and power supply noise which is outside thesignal bandwidth.

The non-linear NRZ level restorer allows higher low-frequency cut-offsto be used throughout the receiver, including the AGC, offsetcompensation and TIA. In this way, the chip area is decreased due tohigher AGC and offset compensation circuit cut-off frequencies, theoff-chip capacitors are not required for compensation circuits,simplifying multi-channel scaling, coding requirements are removed forexternal data, and it allows an AC coupled differential input to beused.

The invention is not limited to optoelectronic applications.Accordingly, in instances where the Rx 300 may be utilized forelectrical communication as opposed to optical communication, the TIA303 may be eliminated or replaced with one or more voltage domainamplifiers.

FIG. 4 is a plot of an input signal and the resulting output signal, inaccordance with an embodiment of the invention. Referring to FIG. 4,there is shown the input signal 401 comprising 10 Gbps NRZ data with anapproximately 159 MHz sinusoidal interferer. The output signal 403 isthe simulated output signal of the optoelectronic receiver 300,demonstrating the large reduction of noise utilizing the filter and NRZlevel restorer.

FIG. 5 is a diagram illustrating performance improvements versus filterand noise frequency, in accordance with an embodiment of the invention.Referring to FIG. 5, there is shown a sensitivity improvement plot 510and a decision threshold plot 503. The x-axis for both plots is theratio, B, of the filter frequency f_(f), to the noise corner frequencyf_(n). The sensitivity improvement plot 501 demonstrates the improvementin sensitivity when the filter frequency approaches that of the noisefrequency, narrowing the bandwidth of the allowed signal. However, asshown by the increase in the normalized decision threshold with B, themagnitude of the input signal required to switch from low to highincreases as the bandwidth narrows, thus demonstrating the tradeoffinvolved in determining the filter cutoff frequency.

FIG. 6 is a diagram illustrating sensitivity penalty at a bit error rate(BER) of 1e-12, in accordance with an embodiment of the invention.Referring to FIG. 6, there is shown a sensitivity penalty versus DCbalance plot 601 and a sensitivity penalty versus ‘0/1’ noise ratio plot603. Due to the DFE-type behavior of this circuit, a single bit flipwill result in errors until another bit flip occurs. For completelyrandom data with equal probabilities of zeros and ones and errors thatare uniformly distributed in time, the increase in average BER due toerror clustering is 50%, assuming run lengths from 1 to infinity areallowed. The plots in FIG. 6, show that the sensitivity penalty is lessthan ˜0.1 dB under practical conditions.

FIG. 7 is a diagram illustrating input and output signals in anarrow-band, non-linear optoelectronic receiver, in accordance with anembodiment of the invention. Referring to FIG. 7, there is shown aninput signal 701, a simulated VGA output signal 703, and a simulated NRZlevel restorer output 705 versus time. The exemplary input signal 701illustrates a square wave signal, and the distorted signal resultingafter the VGA 305, due to non-zero low frequency cut-off, is illustratedby the VGA output signal 703. Because the circuit is effectively ACcoupled to the rest of the system and its feedback maintains the lastvalid bit, the effective low frequency cut-off is OHz. Long runs ofzeros or ones will not produce any pulses and will cause the VGA output703 to squelch, but it will not flip the state of the Circuit outputuntil there is another pulse edge to flip the state of the circuit.

FIG. 8 is a flow diagram illustrating exemplary steps for a narrow-band,non-linear optoelectronic receiver, in accordance with an embodiment ofthe invention. Referring to FIG. 8, in step 803 after start step 801, anoptical signal is received by a photodetector 301 resulting in aphotogenerated current. In step 805 the TIA 303 converts the current toa voltage signal that is amplified in step 807 by the VGA 305 with thegain level controlled by the AGC 307. In step 809, the amplified signalis filtered by the filter 309 and then restored by the NRZ levelrestorer 311, followed by end step 811.

In an embodiment of the invention, a method and system are disclosed foramplifying a received signal, limiting a bandwidth of the receivedsignal, and restoring the amplified, bandwidth-limited received signalutilizing a level restorer 203/311. The level restorer may comprise anon-return to zero (NRZ) level restorer 203/311. The NRZ level restorer203 may comprise a pulse-triggered bistable circuit. The pulse-triggeredbistable circuit 203 may comprise two parallel inverters, and wherein afirst of the inverters comprises a feedback path for a second of theinverters, which may comprise single-ended or differential inverters. Aphotogenerated signal may be amplified in the receiver 300 utilizing atransimpedance amplifier 303 and one or more variable gain amplifiers(VGAs) 305. A received electrical signal may be amplified via one ormore VGAs 305. The bandwidth of the received signal may be limitedutilizing one or more of: a low pass filter, a bandpass filter, a highpass filter 201, a differentiator, or a series capacitance on the chip.The signal may be received from a photodiode 301 integrated on the chip130.

While the invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiments disclosed, but that the present inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method for processing signals, the methodcomprising: in a receiver on a chip, amplifying a received signal thatis generated from a received optical signal; limiting a bandwidth ofsaid received signal utilizing a band pass filter that results in anarrow frequency band comprising a maximum data tone of the receivedsignal; and restoring said received signal utilizing a non-return tozero (NRZ) level restorer comprising a pulse-triggered bistable circuit,wherein said pulse-triggered bistable circuit comprises two parallelinverters, and wherein a first of said inverters comprises a feedbackpath for a second of said inverters.
 2. The method according to claim 1,wherein said inverters comprise single-ended inverters.
 3. The methodaccording to claim 1, wherein said inverters comprise differentialinverters.
 4. The method according to claim 1, comprising amplifying aphotogenerated signal in said receiver utilizing a transimpedanceamplifier and one or more variable gain amplifiers (VGAs).
 5. The methodaccording to claim 1, comprising amplifying a received electrical signalvia one or more VGAs.
 6. The method according to claim 1, comprisinglimiting said bandwidth of said received signal utilizing one or moreof: a differentiator or a series capacitance on said chip.
 7. The methodaccording to claim 1, comprising receiving said signal from a photodiodeintegrated on said chip.
 8. A system for processing signals, the systemcomprising: in a receiver on a chip, one or more circuits operable toamplify a received signal that is generated from a received opticalsignal; said one or more circuits comprise a band pass filter thatlimits a bandwidth of said received signal to a narrow frequency bandthat comprises a maximum data tone of the received signal; and said oneor more circuits restore said received signal utilizing a Non-return toZero (NRZ) level restorer comprising a pulse-triggered bistable circuit,wherein said pulse-triggered bistable circuit comprises two parallelinverters, and wherein a first of said inverters comprises a feedbackpath for a second of said inverters.
 9. The system according to claim 8,wherein said inverters comprise single-ended inverters.
 10. The systemaccording to claim 8, wherein said inverters comprise differentialinverters.
 11. The system according to claim 8, wherein said one or morecircuits are enabled to amplify a photogenerated signal in said receiverutilizing a transimpedance amplifier and one or more variable gainamplifiers (VGAs).
 12. The system according to claim 8, wherein said oneor more circuits are enabled to amplify a received electrical signal viaone or more VGAs.
 13. The system according to claim 8, wherein said oneor more circuits are enabled to limit said bandwidth of said receivedsignal utilizing one or more of: a differentiator or a seriescapacitance on said chip.
 14. The system according to claim 8, whereinsaid one or more circuits are enabled to receive said signal from aphotodiode integrated on said chip.